Method for securing a private key on a mobile device

ABSTRACT

Systems and methods are provided for securing a private key on a mobile device for use with public key cryptography. Specifically, a private key is reduced to two partial keys where the partial keys are stored on separate electronic devices. The partial keys combine to temporarily regenerate the private key for the purposes of notarizing (digitally signing) messages or documents, and decrypting a message or document that was encrypted using the corresponding public key. The partial keys in some embodiments may be a secret key, which can be derived from an account identifier and a password, and an exclusive key, which can be derived from the secret key and the private key. The private key can be regenerated from the secret key and the exclusive key. With the partial keys stored on separate devices, another layer of practical security is provided to public key cryptography.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/853,027, filed Dec. 22, 2017, which is a division of U.S. patentapplication Ser. No. 15/150,764, filed May 10, 2016, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 62/309,602,filed Mar. 17, 2016, the entire disclosures of which are herebyincorporated herein by reference.

FIELD

The invention relates to public key cryptography and the ephemeralregeneration of a private key from multiple, physically separated,partial keys to notarize (i.e. digitally sign), encrypt, or decrypt amessage and improve the security of public key cryptography.

BACKGROUND

Public key cryptography is a type of cryptography that utilizes two keysto notarize a message with a digital signature or encrypt or decrypt amessage. The first key is a public key that is associated with a knownparty, and the public key is available to the public or otherwiseavailable to relevant parties in a given transaction. The second key isa private key that is kept secret by the known party. These two keys aremathematically linked to each other such that it is relatively easy tocalculate the public key from the private key but nearly impossible tocalculate the private key from the public key. Therefore, for example, aparty that receives a message digitally signed with a private key canverify the digital signature with the related public key. However, it isimpractical to mathematically derive the private key from the public keyand then “forge” digital signatures on data or messages using theprivate key. Early examples of public key crypto-systems include RSA andDiffie-Hellman.

While it may be nearly impossible to mathematically calculate a privatekey from a public key, public key cryptography is still susceptible tonon-mathematical security issues, for example, conventional theft of theprivate key. Thus, the security of public key cryptography hinges on theability of the party to keep the private key secret and confidential.Physical theft of a notebook computer or other electronic device,hacking into an email account, malware, etc. are all security issuesthat routinely occur and compromise public key cryptography. Therefore,there is a need to provide better security for the private key in publickey cryptography where the theft of a device or the compromise of adevice will not allow the thief to possess the private key andcompromise the public key cryptography.

SUMMARY

Embodiments of the invention promote the security of a private key byreducing the private key into multiple partial keys, which are stored onphysically separate electronic devices. Therefore, if one electronicdevice is stolen or compromised, then the partial key on that devicecannot be used to forge a digital signature on a message, a document, ordata. Instead, the multiple partial keys are assembled to temporarilyregenerate the private key only with explicit authorization from theowner. After using the regenerated private key to notarize, encrypt, ordecrypt a message, the regenerated private key is deleted, and onceagain, only partial keys exist on the physically separate devices.

In accordance with embodiments of the present invention, a cryptographichash function HASH is used to generate a fixed length bit string fromany data string, like an account identifier or an account password. Acryptographic hash function can be chosen that generates a bit stringwith the same number of bits as a private key. Also in accordance withembodiments of the present invention, the exclusive disjunctionfunction, commonly referred to as “exclusive-or” or XOR, is utilized toconstruct a key from two partial keys. Each key is a bit string of thesame length as the output of the cryptographic hash function. Theexclusive-or function is a boolean function that operates on each pairof bits in the input strings and outputs a 1 bit only when thecorresponding bits in each input differ. For example: 1001 XOR 1100yields 0101. Using the cryptographic hash function and the exclusive-orfunction a secret key S with the same number of bits as the private keyk may be generated from a login account identifier A and an accountpassword P as follows: S=XOR(HASH(A), HASH(P)). Then an exclusive key Xcan be generated from the secret key S and the private key k using theexclusive-or function as follows: X=XOR(S, k). As a result, the secretkey and the exclusive key are partial keys, and the exclusive-or of thesecret key and the exclusive key reproduces the private key as follows:k=XOR(X, S).

The partial keys should be stored on physically separate devices toimprove the security of public key cryptography. In some embodiments,the secret key may be stored on a mobile device such as a smart phone, atablet computer, or a notebook computer; and the exclusive key may bestored on a wearable device such as a ring, a watch, a necklace, abracelet, or any other similar type of article that is distinct from themobile device. The user logs into the mobile device, which stores thesecret key, and the user wears the wearable device, which stores theexclusive key. Generally, when the user wishes to notarize, encrypt, ordecrypt a message, the mobile device sends the secret key to thewearable device. Then, the wearable device regenerates the private keyby taking the exclusive-or of the secret key and the exclusive key,which is stored on the wearable device. The mobile user can then beprompted by the wearable device to press a button on the wearable deviceor otherwise authorize the wearable device to use the regeneratedprivate key for the intended purpose. After serving its purpose, thewearable device deletes the regenerated private key and the secret keyand retains the exclusive key. Therefore, if the mobile device or thewearable device is stolen, it is impossible to regenerate the privatekey and forge a digital signature using that private key.

The combination of the mobile device and the wearable device thatreceives authorization input from a mobile user can be used to performvarious operations in the present invention. For instance, the mobileuser may press and hold a button on the wearable device to authorize thepairing of the wearable device and the mobile device. Once paired, themobile user may press the button on the wearable device to notarize amessage, encrypt a message, or decrypt a message. The notarized and/orencrypted message can then be used to log the mobile user into his orher mobile device, computer, or a remote web site, send a digitalpayment to a merchant, digitally sign (notarize) a legal contract, andmany other nonrepudiateable transactions. Further, the mobile user canuse the button on the wearable device to authorize the resetting of apassword on the mobile device or the rotation of the private and publickeys to maintain the integrity of the public key cryptography system.

It will be appreciated that embodiments of the present invention mayextend beyond the wearable device and the mobile device describedherein. For example, physically separate servers may store partial keysto guard against the theft or compromise of one of the servers. Theservers may combine the partial keys to temporarily regenerate a privatekey for various public key cryptography purposes. Thus, the wearabledevice and the mobile device may generally refer to a first electronicdevice and a second electronic device. Further still, embodiments of thepresent invention may be performed on a single electronic device. Inthis sense, physically separate means that the partial keys are stored,for example, on different hard drives or storage devices, or differentportions of the same hard drive or storage device such that the partialkeys are compartmentalized. Thus, the security compromise of one partialkey does not affect the security of the other partial key, and theprivate key can be temporarily regenerated from the partial keys.

Additional features and advantages of embodiments of the presentdisclosure will become more readily apparent from the followingdescription, particularly when taken together with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a use case diagram of a system for regenerating a private keythat depicts the various operations of the system in accordance withembodiments of the invention;

FIG. 2 is a sequence diagram that depicts pairing a mobile electronicdevice and a wearable electronic device in accordance with embodimentsof the invention;

FIG. 3 is a sequence diagram that depicts registering a new account fora mobile user in accordance with embodiments of the invention;

FIG. 4 is a sequence diagram that depicts logging into a mobile deviceto use the mobile device in combination with the wearable device inaccordance with embodiments of the invention;

FIG. 5 is a sequence diagram that depicts notarizing a message with aregenerated private key in accordance with embodiments of the invention;

FIG. 6 is a sequence diagram that depicts encrypting a message with apublic key in accordance with embodiments of the invention;

FIG. 7 is a sequence diagram that depicts decrypting a message with aregenerated private key in accordance with embodiments of the invention;

FIG. 8 is a sequence diagram that depicts adding another mobile devicethat operates in combination with the wearable device in accordance withembodiments of the invention;

FIG. 9 is a sequence diagram that depicts resetting a password for themobile device in accordance with embodiments of the invention;

FIG. 10 is a sequence diagram that depicts rotating the private key andthe public key of the system in accordance with embodiments of theinvention;

FIG. 11A is a block diagram of a system having a first device and asecond device in accordance with embodiments of the invention; and

FIG. 11B is a block diagram of a system having first, second, and thirddevices in accordance with embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1 depicts a system 100 for regenerating a private key for publickey cryptography using two partial keys stored on physically separatedevices. The diagram illustrates how a mobile user 104 can set up thesystem 100, utilize the system 100 for encryption and decryption, andperform other various functions with a regenerated private key. Themobile user 104 can set up the system 100 by pairing 108 a mobile deviceand a wearable device, registering 112 as a new user, and then logging116 into the mobile device. Once set up is complete, the mobile user 104can utilize the system 100 to digitally sign or notarize 120 messageswith a regenerated private key, encrypt 124 messages to another mobileuser or system using the other user or system's public key, and decrypt128 messages using the regenerated private key. The mobile user 104 canalso perform functions such as adding 132 another mobile device thatoperates with the wearable device, resetting 136 the password to themobile device, and rotating 140 the private key to maintain the securityof public key cryptography. Embodiments of the present invention aredepicted as having two electronic devices that regenerate a private keyfrom two partial keys where one partial key is stored on the firstelectronic device and another partial key is stored on the secondelectronic device. However, it will be appreciated that embodiments ofthe present invention may include only one electronic device that canstore the partial keys in physically separate locations or more than twoelectronic devices.

FIG. 2 depicts a sequence for pairing 108 a mobile device 148 with awearable device 144 so that the devices 144, 148 may operate together todigitally sign messages or decrypt messages using a regenerated privatekey. Generally, the mobile user 104 will log into the mobile device 148,and then provide an identifier for the wearable device 144 to the mobiledevice 148 to initiate the pairing 108. To prompt the mobile user 104for his or her authorization of the pairing 108, the wearable device 144may emit a light or sound or produce haptic feedback. The wearabledevice 144 may also have features to receive an input from the mobileuser 104 including, but not limited to, a button, an accelerometer, avoice recognition system, a fingerprint scanner, a keypad, a touchscreen, etc. Thus, a wearable device 144 may flash a light to indicateto the next step in a sequence is ready to be performed, and the mobileuser 104 may press a button on the wearable device 144 to authorize thenext step in the sequence. The wearable device 144 may be a ring,bracelet, necklace, or other articles of clothing that people commonlywear, or even a human implantable device in some embodiments.

In the depicted sequence 108, the wearable device 144 is manufactured152 with a unique device identifier D, which could be a bit string, a QRcode, a barcode, etc. The mobile user 104 enters 156 the deviceidentifier D into the mobile device 148, and the mobile device 148enters 160 into a pairing mode. In this mode, the mobile device 148requests 164 that the mobile user 104 enter an input into the wearabledevice 144 to initiate a pairing request with the mobile device 148. Asnoted above, the wearable device 144 may have a button, and the mobileuser 104 satisfies the request 164 by pressing and holding 168 thebutton on the wearable device 144.

While the mobile user 104 holds the button, the wearable device 144sends 172 a pairing request to the mobile device 148, and the mobiledevice 148 returns 176 the device identifier D to the wearable device144. Then, the wearable device 144 verifies 180 that the deviceidentifier D received from the mobile device 148 matches the deviceidentifier D of the wearable device 144. Next, the wearable device 144exchanges 184 an encryption key with the mobile device 148 to securefuture communication between the two devices 144, 148, and the mobiledevice 148 returns 188 a confirmation of the encryption key exchangewith the wearable device 144. The encryption key can be used toestablish a secure communication protocol between the devices 144, 148,including, but not limited to, Bluetooth 4, and near fieldcommunications (NFC) with transport layer security (TLS) 1.2 or greater.

The wearable device 144 sends 192 an indication of a successful pairingof the devices 144, 148, which could be a light, flashing or otherwise,on an exterior of the wearable device 144. The mobile user 104 thenreleases 196 the button, and the wearable device 144 turns 200 off thelight. With the devices 144, 148 successfully paired, the devices 144,148 may securely exchanged information to perform the functionsdescribed herein.

The pairing 108 of the devices 144, 148 through a wireless communicationprotocol adds to the security function of public key cryptographybecause there is a maximum operational distance between the devices 144,148. The maximum range for some low energy Bluetooth devices isapproximately 50 meters. Thus, if one of the devices 144, 148 is stolenand transported a greater distance than the maximum operational range ofthe wireless communication protocol between the devices 144, 148, thenit is impossible to regenerate the private key to forge digitalsignatures, etc.

FIG. 3 shows a sequence for registering 112 a new mobile user 104 toestablish a new mobile user account associated with a new public-privatekey pair and to store the account information including the public keyin an identity registry 202. First, the public-private key pair isestablished. In the depicted embodiment, the mobile user 104 enters 204an email address E and a new password P into the mobile device 148,which generates 208 a new random account identifier A for the mobileuser 104. Next, the mobile device 148 derives 212 a secret keyS=XOR(HASH(A), HASH(P)) using a cryptographic hash function and abit-wise exclusive-or function. A cryptographic hash function HASH is afunction that maps data of an arbitrary size, for example, anarbitrarily long account identifier A or password P, to a fixed sizecrypto-random bit string. The bit-wise exclusive-or function XOR is aboolean function that operates on each pair of bits in the input stringsand outputs a 1 bit only when the corresponding bits in each inputdiffer and a 0 bit otherwise. For example: 1001 XOR 1100 yields 0101.Therefore, when the hash of the account identifier A and the hash of thepassword P are the same size, then the result of the exclusive-or of thetwo hashes, the secret key S, is also the same size.

In addition, it will be appreciated that the exclusive-or of the hash ofthe account identifier A and the hash of the password P may be, in someembodiments, any one of many approaches, including (1) an exclusive-orof the concatenation of the account identifier and the password P, (2)an exclusive-or of the concatenation of the email address E, thepassword P and a random salt value R, or (3) an exclusive-or of thepassword based encryption of the email address E. It will be furtherappreciated that these operations and other operations described hereinmay be commutative.

Referring again to FIG. 3, after deriving the secret key S, the mobiledevice 148 forgets or deletes 216 the password P from a non-transitorycomputer-readable storage medium on the mobile device 148. Examples ofnon-transitory computer-readable storage mediums may include volatilememory such as random access memory and non-volatile memory such assolid state drives or hard disk drives. Both devices 144, 148 may havenon-transitory computer-readable storage media to store and deletevarious keys and data as necessary. Next, the mobile device 148initializes 220 the wearable device 144 by sending the wearable device144 the account identifier A, the email address E, and the secret key S.The wearable device 144 turns on 224 a light, and the mobile user 104authorizes 228 the initialization by pressing a button on the wearabledevice 144 which turns off the light on the wearable device 144. Then,the wearable device 144 generates 232 a new private key k andcorresponding public key K that can be used to verify data or messagesthat have been notarized, or digitally signed using the private key k.The wearable device 144 derives 236 an exclusive key X=XOR(S, k) whichis a partial key like the secret key S. From now on the private key kcan be regenerated using the exclusive-or function with k=XOR(X, S). Thewearable device 144 then stores 240 the public key K and the exclusivekey X, on a non-transitory computer-readable storage medium.

Next, the wearable device 144 creates 244 a public certificate Ccontaining the account identifier A, the email address E, and the publickey K. The wearable device 144 notarizes 248 the public certificate Cusing the private key k to add a notary seal which contains the digitalsignature of the contents of the public certificate C. The digitalsignature can be verified by any party using the public key K that isnow contained in the public certificate C. Thus, the notary seal allowsmessage sender authentication and non-repudiation. The wearable device144 forgets or deletes 252 the private key k and the secret key S from anon-transitory computer-readable storage medium. The wearable device 144returns 256 the notarized public certificate C to the mobile device 148where the mobile device 148 verifies 260 the notary seal on the publiccertificate C using the public key K. If the notary seal is valid themobile device 148 stores 264 the secret key S and the public certificateC in a non-transitory computer-readable storage medium.

Now that the wearable device 144 has notarized the public certificate C,the mobile device 148 registers 268 a new account A with the identityregistry 202, and the new account A is associated with the mobile user'semail address E and the notarized public certificate C. The identityregistry 202 verifies 272 the notary seal of the public certificate Cwith the public key K. The identity registry 202 then associates 276 thenew account with a temporary unique token T, and the identity registry202 creates 280 the new account with a pending status.

To change the account status to active, the mobile user 104 must confirmhis or her identity with the identity registry 202. The identityregistry 202 sends 284 a confirmation to the mobile user 104, which maybe an email with a confirmation link. The identity registry 202 alsosends 288 a confirmation that the new account A was created to themobile device 148. In turn, the mobile device 148 returns 292 aconfirmation to the mobile user 104 to check for the email confirmationlink. The mobile user 104 selects 296 the email confirmation link, whichcontains the unique token T associated with the new account. Theidentity registry 202 receives the message containing the unique token Tand retrieves 300 the account associated with the unique token T, andthe identity registry 202 changes 304 the status of the account frompending to active. Lastly, the identity registry 202 returns 308 aconfirmation of success to the mobile user 104.

FIG. 4 depicts a sequence allowing the mobile user 104 to login 116 tothe mobile device 148 by using the combination of the mobile device 148and the wearable device 144 to create a login notary seal containing anunforgeable digital signature. To begin, the mobile user 104 wakes up340 the mobile device 148 by, for example, pressing a touch screen onthe mobile device 148, or opening the cover on the mobile device 148.The mobile device 148 initiates 344 the login sequence by passing thesecret key S to the wearable device 144. The wearable device 144regenerates 348 the private key k by taking the exclusive-or of theexclusive key X (stored on the wearable device 144) and the secret key S(supplied by the mobile device 148) as k=XOR(X, S). The wearable device144 then verifies 352 the regenerated private key k with the public keyK. The wearable device 144 turns on 356 a light, and the mobile user 104confirms 360 that the mobile user 104 wants to login by pressing abutton on the wearable device 144 which turns off the light on thewearable device 144. It will be appreciated that in some embodiments, abit-wise exclusive-or function XOR may instead be any one of an advancedencryption standard, a blowfish cipher, a triple DES cipher, and a XORcipher.

After authorization from the mobile user 104, the wearable device 144notarizes 364 the secret key S using the regenerated private key k tocreate a notary seal N. Then, the wearable device 144 forgets or deletes368 the regenerated private key k from a non-transitorycomputer-readable storage medium. The wearable device 144 sends 372 thenotary seal N to the mobile device 148. The mobile device 148 verifies376, using the public key K (contained in the public certificate Cstored on the mobile device 148), that the notary seal N corresponds tothe secret key S (also stored on the mobile device 148) and, if so, themobile device 148 unlocks 380 for the mobile user 104.

Once logged into the mobile device 148, the mobile user 104 can performa variety of cryptographic functions using the wearable device 144 toregenerate the private key. FIG. 5 depicts a sequence for notarizing 120a message or electronic document with a digital signature using aregenerated private key. The mobile user 104 requests 384 the mobiledevice 148 notarize a message M. The mobile device 148 sends 388 themessage M with the secret key S to the wearable device 144. The wearabledevice 144 regenerates 392 the private key k by taking the exclusive-orof the exclusive key X (stored on the wearable device 144) and thesecret key S (supplied by the mobile device 148) as k=XOR(X, S). Thewearable device 144 turns on 400 a light, and the mobile user 104authorizes 404 the notarization by pressing a button on the wearabledevice 144 which turns off the light on the wearable device 144.

Next, the wearable device 144 notarizes 408 the message M using theregenerated private key k to create a notary seal N, and the wearabledevice 144 forgets or deletes 412 the regenerated private key k from anon-transitory computer-readable storage medium. The wearable device 144returns 416 the notary seal N to the mobile device 148, and the mobiledevice 148 verifies 420, using the public key K (contained in the publiccertificate C stored on the mobile device 148), that the notary seal Ncorresponds to the message M. After verification, the mobile device 148returns 424 a confirmation of success to the mobile user 104. Thisindicates that the message M has been successfully notarized, and anyparty with the public key K can verify the notary seal N of the messageM to confirm that the mobile user 104 notarized the message M using theprivate key k.

FIG. 6 depicts a sequence for encrypting 124 a message that is to besent to another mobile user or remote system, using the public key K ofthe other mobile user or remote system. First, the mobile user 104requests 428 that the mobile device 148 encrypt a message M for accountA. In turn, the mobile device 148 requests 432 from the identityregistry 202 the identity information contained in a public certificateC associated with account A. The identity registry 202 retrieves 436 therequested information, and the identity registry 202 returns 440 thepublic certificate C to the mobile device 148. As explained elsewhereherein, the public certificate C contains the account identifier A, theemail address E associated with the account, the public key K associatedwith the private key k that was used to notarize the public certificateC for the account. The mobile device 148 verifies 444 the publiccertificate C using the public key K and that the account identifiercontained in the public certificate C matches the account identifier Arequested. Then, the mobile device 148 encrypts 448 the message M withthe public key K and returns 452 a confirmation of success to the mobileuser 104. Now the encrypted message M can only be decrypted by the ownerof the account A using the private key k associated with account A.

FIG. 7 depicts a sequence for decrypting 128, using a regeneratedprivate key, an encrypted message M that was received from anothermobile user or remote system. Other parties can encrypt a message mprior to sending it to the mobile user 104 using the public key Kcontained in the public certificate C that is available from theidentity registry 202 as previously described. First, the mobile user104 requests 456 that the mobile device 148 decrypt a message M, and themobile device 148 sends 460 the message M with the secret key S to thewearable device 144. The wearable device 144 regenerates 464 the privatekey k by taking the exclusive-or of the exclusive key X (stored on thewearable device 144) and the secret key S (supplied by the mobile device148) as k=XOR(X, S). Then, the wearable device 144 verifies 468 theregenerated private key k using the public key K. The wearable device144 turns on 472 a light, and the mobile user 104 authorizes 476 theencryption by pressing a button on the wearable device 144 which turnsoff the light on the wearable device 144.

With the authorization from the mobile user 104, the wearable device 144decrypts 480 the message M using the regenerated private key k. Then,the wearable device 144 forgets 484 or deletes the regenerated privatekey k from a non-transitory computer-readable storage medium. Thewearable device 144 returns 488 the decrypted message m to the mobiledevice 148, and the mobile device 148 sends 492 a confirmation ofsuccess to the mobile user 104, which in this embodiment may be thedisplay of the unencrypted message m on a display unit of the mobiledevice 148.

FIG. 8 depicts a sequence for adding 132 a new mobile device 496 thatuses the existing wearable device 144 to regenerate a private key.First, the mobile user 104 enters 500 into the new mobile device 496 theemail address E and password P associated with the existing accountidentifier A for the mobile user 104. The new mobile device 496 requests504 the identity information associated with the email address E fromthe identity registry 202, which in turn, retrieves 508 the identityinformation. The identity registry 202 then returns 512 the publiccertificate C containing the identity information to the new mobiledevice 496. The new mobile device 496 verifies 516 the publiccertificate C using the public key K contained in the public certificateC, and that the email address contained in the public certificate Cmatches the email address E requested by the mobile user 104. If thepublic certificate C and email address E are valid, the new mobiledevice 496 derives 520 the secret key S using the account identifier Aand the password P as S=XOR(HASH(A), HASH(P)), described previously, andthe new mobile device 496 forgets or deletes 524 the password P from anon-transitory computer-readable storage medium.

Now that the new mobile device 496 has the secret key S, the new mobiledevice 496 sends 528 the secret key S to the wearable device 144, andthe wearable device 144 regenerates 532 the private key k by taking theexclusive-or of the exclusive key X (stored on the wearable device 144)and the secret key S (supplied by the new mobile device 496) as k=XOR(X,S). The wearable device 144 then verifies 536 the private key k usingthe public key K. The wearable device 144 turns on 540 a light, and themobile user 104 authorizes 544 the addition on the new mobile device bypressing a button on the wearable device 144 which turns off the lighton the wearable device 144. With authorization from the mobile user 104,the wearable device 144 notarizes 548 the secret key S using theregenerated private key k to create a notary seal N. Then, the wearabledevice 144 forgets or deletes 552 the regenerated private key k from anon-transitory computer-readable storage medium.

The wearable device 144 returns 556 the notary seal N for the secret keyS to the new mobile device 496. The new mobile device 496 verifies 560the notary seal N using the secret key S and the public key K, and thenew mobile device 496 remembers or stores 564 the public certificate Cand the secret key S on a non-transitory computer-readable storagemedium. Then, the new wearable device 496 sends 568 a confirmation ofsuccess to the mobile user 104, indicating that the new mobile device496 can now operate with the wearable device 144 to notarize, encryptand decrypt messages or data.

FIG. 9 depicts a sequence for resetting 136 the password for the mobiledevice 148. The mobile user 104 enters 572 the existing password P and anew password P′ into the mobile device 148. First, the mobile device 148verifies 576 the existing password P by comparing the secret key S toXOR(HASH(A), HASH(P)), where the account identifier A is contained inthe public certificate C that is stored by the mobile device 148. If thesecret key S matches, the mobile device 148 derives 580 a new secret keyS′=XOR(HASH(A), HASH(P′)) using the new password P′. Afterwards, themobile device forgets or deletes 584 the old password P and the newpassword P′ from a non-transitory computer-readable storage medium.

The mobile device 148 sends 588 the new secret key S′ to the wearabledevice 144 to replace the existing secret key S. The wearable device 144regenerates 592 the private key k by taking the exclusive-or of theexisting exclusive key X (stored on the wearable device 144) and theexisting secret key S (supplied by the new mobile device 496) ask=XOR(X, S), and then verifies 596 the regenerated private key k usingthe public key K. The wearable device 144 turns on 600 a light, and themobile user 104 authorizes 604 the password change by pressing a buttonon the wearable device 144 which turns off the light on the wearabledevice 144. The wearable device 144 derives 608 a new exclusive keyX′=XOR(S′, k) and stores the new exclusive key X′ in a non-transitorycomputer-readable storage medium. The wearable device 114 then forgetsor deletes the old exclusive key X and the old secret key S from anon-transitory computer-readable storage medium.

The wearable device 144 notarizes 612 the new secret key S′ using theregenerated private key k to create a notary seal N, and then forgets ordeletes 616 the regenerated private key k and the new secret key S′ froma non-transitory computer-readable storage medium. Then the wearabledevice 144 returns 620 the notary seal N for the new secret key S′ tothe mobile device 148. The mobile device 148 verifies 624 the notaryseal N using the public key K. Then, the mobile device 148 remembers orstores 628 the new secret key S′ on a non-transitory computer-readablestorage medium, and the mobile device 148 sends 632 a confirmation ofsuccess to the mobile user 104, indicating that the password has beenreset.

FIG. 10 depicts a sequence for rotating 140 the private key k. While itis highly unlikely to calculate or derive the private key k from thepublic key K, the more times that the private key k has beencumulatively used, the greater the likelihood that the private key kcould be derived from the public key K and compromise the security ofthe system. Therefore it is desirable to periodically rotate keys(replace old keys with new keys) to guard against this aspect of publickey cryptography. To begin the key rotation 140, the mobile user 104requests 636 that the mobile device 148 rotate the current key pair.

Next, the mobile device 148 sends 640 the public certificate C and thesecret key S to the wearable device 144. The wearable device 144regenerates 644 the private key k by taking the exclusive-or of theexisting exclusive key X (stored on the wearable device 144) and theexisting secret key S (supplied by the new mobile device 496) ask=XOR(X, S), and then the wearable device 144 verifies 648 theregenerated private key k using the public key K. The wearable device144 turns on 652 a light, and the mobile user 104 authorizes 656 the keyrotation by pressing a button on the wearable device 144 which turns offthe light on the wearable device 144. The wearable device 144 generates660 a new private key k′ and corresponding public key K′ that can beused to verify any notarizations done with the new private key k′. Thewearable device 144 derives 664 a new exclusive key X′=XOR(S, k′). Thewearable device 144 stores 668 the new public key K′ and new exclusivekey X′ on a non-transitory computer-readable storage medium. Then, thewearable device 144 forgets or deletes 672 the old public key K, the oldprivate key k, and the secret key S from a non-transitorycomputer-readable storage medium.

To complete the key rotation, a new public certificate C′ must becreated. The wearable device 144 generates 676 a new public certificateC′ containing the account identifier A, the email address, and the newpublic key K′. The wearable device 144 notarizes 680 the new publiccertificate C′ using the newly generated private key k′ to create anotary seal containing a digital signature of the contents of the newpublic certificate C′. Then, the wearable device 144 forgets or deletes684 the newly generated private key k′ from a non-transitorycomputer-readable storage medium. The wearable device 144 returns 688the new notarized public certificate C′ to the mobile device 148, andthe mobile device 148 verifies 692 the new notarized public certificateC′ using the new public key K′. The mobile device 148 registers 696 thenew public certificate C′ for the account identifier A with the identityregistry 202, which verifies 700 the new public certificate C′ using thenew public key K′ and stores 704 the new public certificate C′ on anon-transitory computer-readable storage medium. The identity registry202 returns 708 a confirmation of success to the mobile device 148, andthe mobile device 148 remembers or stores 712 the new public certificateC′ on a non-transitory computer-readable storage medium. The mobiledevice 148 forgets or deletes 716 the old public certificate C from anon-transitory computer-readable storage medium. Then, the mobile device148 returns 720 a confirmation of success to the mobile user 104 tocomplete the sequence.

FIGS. 11A and 11B depict block diagrams of systems 724, 772 thatimplement the methods and processes described herein. The first system724 comprises physically separate devices: a wearable device 144 and amobile device 148. The wearable device 144 has a processor 728, memory732 with data 736 and an application 740, and an input/output (I/O) port744. The memory 732 can store data 736, for example, the exclusive key Xdescribed herein, and the memory 732 can also store an application 740with instructions for manipulating data 736 and communicating with othercomponents of the wearable device 144. The processor 728 is configuredto execute instructions from the application 740 and the memory 732.Lastly, the I/O port 744 can transmit and receive information fromoutside of the wearable device 144 to complete the methods and processesdescribed herein.

Similarly, the mobile device 148 also has a processor 748, memory 752with data 756 and an application 760, and an I/O port 764. Further, thecomponents of the mobile device 148 function like the components of thewearable device 148 with obvious exceptions, for example, the secret keyS is data 756 stored on the memory 752 of the mobile device 148.

The devices 144, 148 of the first system 724 are physically separate,but the devices 144, 148 communicate with each other via theirrespective I/O Ports 744, 764 and a communication protocol 768. Asdescribed above, a secure communication protocol is desired between thedevices 144, 148, and examples of the communication protocol 768include, but are not limited to, Bluetooth 4 and near fieldcommunications (NFC) with transport layer security (TLS) 1.2 or greater.

Now referring to FIG. 11B, the second system 772 comprises a wearabledevice 144, a mobile device 148, and an identity registry 202. Thewearable device 144 and the mobile device 148 have the same componentsas described above with respect to FIG. 11A, and similarly, the identityregistry 202 has a processor 776, memory 780 with data 784 and anapplication 788, and an I/O port 792. The devices 144, 148 and theregistry 202 are all physically separate, and the identity registry 202is connected to one or both of the devices 144, 148 via the I/O port 792and a communication protocol 796. In various embodiments, thecommunication protocols 768, 796 may be the same, and in someembodiments, the communication protocols 768, 796 may be distinct.

Accordingly, the invention has been described with some degree ofparticularity directed to the exemplary embodiments of the invention. Itshould be appreciated though that modifications or changes may be madeto the exemplary embodiments of the present invention without departingfrom the inventive concepts contained herein.

As set forth below, the claims generally, but not exclusively, describemethods and systems for making public-private key cryptography moresecure by splitting a private key k into two partial keys across twodifferent electronic devices.

What is claimed is:
 1. A method for securing a private key k,comprising: providing a first electronic device and a second electronicdevice; pairing the first electronic device and the second electronicdevice via a wireless communication protocol; generating a private key kon the second electronic device, wherein the private key k is associatedwith a public key K; and splitting the private key k into a firstpartial key and a second partial key, wherein the first partial key isstored on the first electronic device, and the second partial key isstored only on the second electronic device such that the private key kcan only be reconstructed on the second electronic device using bothpartial keys.
 2. The method of claim 1, further comprising: activatingan input feature of the first electronic device to generate the firstpartial key; transmitting the first partial key from the firstelectronic device to the second electronic device; and activating aninput feature of the second electronic device to generate the privatekey k associated with the public key K and to derive a second partialkey from the first partial key and the private key k to split theprivate key k.
 3. The method of claim 2, wherein the input feature ofthe first electronic device is at least one of a button, anaccelerometer, a voice recognition system, a biometric scanner, a keypad, a touch screen, or a microphone, and wherein the input feature ofthe second electronic device is at least one of a button, anaccelerometer, a voice recognition system, a biometric scanner, a keypad, a touch screen, or a microphone.
 4. The method of claim 2, furthercomprising: activating, after transmitting the first partial key, anoutput feature of the second electronic device to prompt a user toactivate the input feature of the second electronic device.
 5. Themethod of claim 4, wherein the output feature of the second electronicdevice is at least one of a light, an LED, a sound device, or hapticfeedback device.
 6. The method of claim 1, wherein the first electronicdevice is a mobile device, and the second electronic device is awearable device.
 7. The method of claim 1, further comprising: ceasingthe pairing between the first electronic device and the secondelectronic device via the wireless communication protocol when the firstelectronic device and the second electronic device are separated by adistance greater than a maximum operational distance.
 8. The method ofclaim 1, further comprising: deleting the private key k from the secondelectronic device.
 9. A method for reconstructing a private key k,comprising: providing a mobile device and a wearable device; pairing themobile device and the wearable device via a wireless communicationprotocol; transmitting a first partial key from the mobile device to thewearable device; reconstructing a private key k from the first partialkey and a second partial key; performing, by the wearable device, acryptographic operation using the reconstructed private key k; anddeleting the private key k and the first partial key from the wearabledevice.
 10. The method of claim 9, wherein the wearable device has aninput feature, and activation of the input feature causes the wearabledevice to reconstruct the private key k.
 11. The method of claim 9,wherein an exclusive-or (XOR) operation on the partial keys reconstructsthe private key k on the wearable device.
 12. The method of claim 9,wherein a secret sharing algorithm using the partial keys reconstructsthe private key k on the wearable device.
 13. The method of claim 9,wherein a symmetric key decryption of one partial key using the otherpartial key reconstructs the private key k on the wearable device. 14.The method of claim 9, wherein the cryptographic operation includesdigitally signing a stream of bytes.
 15. The method of claim 9, whereinthe cryptographic operation includes decrypting a stream of bytes thatwas encrypted using a public key K associated with the private key k.16. A method for initiating the use of the private key k, comprising:providing a mobile device and a wearable device, wherein the mobiledevice has an input feature and the wearable device has an outputfeature; pairing the mobile device and the wearable device via awireless communication protocol; activating the input feature of themobile device to initiate the use of a private key k on the wearabledevice, wherein the private key k is associated with a public key K; andprompting, by the output feature of the wearable device, a user toapprove the use of the private key k for a cryptographic operation. 17.The method of claim 16, wherein the output feature prompts a user toapprove the reconstruction of the private key k on the wearable deviceusing a plurality of partial keys.
 18. The method of claim 16, whereinthe wearable device has an input feature, and activation of the inputfeature of the wearable device approves the use of the private key k forthe cryptographic operation.
 19. The method of claim 16, wherein the useof the private key k for the cryptographic operation is to digitallynotarize (sign) one of a document, a message, or a legal contract. 20.The method of claim 16, wherein the use of the private key k for thecryptographic operation is to decrypt one of a document, a message, or afile.